Since the method for synthesizing a carbon cluster (hereinafter also referred to as “fullerene”), in which carbon atoms are arranged to form a spherical shape or a rugby ball shape, was established, fullerene has been energetically studied. As a result, many fullerene derivatives have been synthesized.
With respect to specific examples of such fullerene derivatives, methods for synthesizing a fullerene derivative, in which 5 organic groups bind to a fullerene skeleton (hereinafter also just referred to as “penta(organo)fullerene derivative”), have been reported (e.g., Japanese Laid-Open Patent Publication No. Hei 10-167994; Japanese Laid-Open Patent Publication No. Hei 11-255509; J. Am. Chem. Soc., 118, 12850 (1996); Org. Lett., 2, 1919 (2000); and Chem. Lett., 1098 (2000)).
As a method for producing a penta(organo)fullerene derivative, for example, it is known that, by reacting an organocopper reagent prepared using a phenyl Grignard reagent and CuBr.S(CH3)2 with fullerene C60, a phenylated fullerene derivative, in which phenyl groups constituting the phenyl Grignard reagent are regioselectively added to surround one 5-membered ring of fullerene C60 (C60Ph5H), can be quantitatively obtained (e.g., Japanese Laid-Open Patent Publication No. 10-167994).
However, since compounds having a substituent such as a carboxyl group and an ester group are active against Grignard reagents, it is difficult to prepare Grignard reagents having such a substituent. Therefore, it has been impossible to use methods for conveniently synthesizing a fullerene derivative having a substituent such as a carboxyl group and an ester group using a Grignard reagent having such a substituent.
As a result, in order to synthesize a fullerene derivative having such a substituent, it has been necessary to use time-consuming methods such as: methods for producing a penta(organo)fullerene derivative by reacting a large excess of bromomalonate derivative with fullerene in a multistage manner (Angew. Chem. Int. Ed. Engl., 33, 2339 (1994); Angew. Chem. Int. Ed. Engl., 34, 1607 (1995); etc.); and a multistage synthesis method comprising the step of reacting C60I6 with benzene to cause electrophilic substitution (J. Chem. Soc., Chem. Commun. 1464 (1994)); etc. In addition, in the case of these methods for producing fullerene derivatives, there are problems that it is very difficult to regioselectively obtain a penta(organo)fullerene derivative, and that the yield of fullerene derivative is low.